This invention relates in general to packaging material and to the packages formed from such material for use in packaging electronic and medical products where it is important that static charge buildup on the interior of the package be avoided.
The problem with packaging electronic components and units so as to avoid the affects of electrostatic discharge has become increasingly acute as smaller and smaller dimensions are achieved in integrated circuits. The buildup of electrostatic charges on the packages for these components and units can result in the discharge of a spark which will destroy the micro-electronic chip, printed circuit or other electronic unit. For example, a static charge of as little as 100 volts can discharge through a 1000 Angstrom thick gate oxide to cause a short and to render a MOS device nonfunctional.
Similarly ultra pure chemicals and drugs used in both electronic and medical industries have severe restrictions on the amount of foreign particulate matter which can be present. Packaging these chemicals poses severe problems because an electrostatic charged surface will attract charged particulate matter in the air and cause contamination.
Accordingly in a wide range of industry, from aero space and electronic to medical and drug, the prevention and control of static charge buildup on the packages in which goods are packed and shipped is a major goal.
There are known anti-static packages and packaging material to prevent and minimize static buildup. In one approach, a plastic film or sheet is compounded with an anti-static additive such as a quaternery amine, amine salts or soaps which gives a product that will not build up a static charge. Packages and bags formed of such material have been used. A major drawback for such packages is that this design provides no shielding from external static fields.
One technique for grounding electrostatic charges brought into contact with the envelope or packages is described in U.S. Pat. No. 4,154,344 and No. 4,156,751 both issued to Yenni et. al. These patents teach a design in which a flexible substrate of a polyester film is coated on one side with an anti-static polyethylene material and on the other side with a very thin layer of nickel. The anti-static ply defines the interior of the package. The metal ply is on the exterior of the package. Because it is a metal layer, electrostatic charge will not build up. The nickel is vacuum deposited and is thin enough to be partially transparent. However, the optical transmission of the nickel layer in the visual spectrum is sufficiently poor to hinder visual inspection of stored items. Furthermore, when the package is flexed, particulate matter tends to flake off from the very thin nickel layer and may contaminate clean room environments. A very thin abrasion resistant layer on the outside of the nickel ply is not adequate to prevent this particulate matter flake-off. Furthermore, in certain environments where electronic components are taken out of the package for testing and are replaced, the external or substantially external metal ply provides a conductive path and contact with an electric terminal may provide a source of electrical shock to the testing personnel.
Accordingly, it is a purpose of this invention to provide an improved anti-static sheet material which in addition to providing the anti-static benefits of known prior art devices will provide improved ability to visually inspect the products packaged, an enhanced safety feature against conductive shock and reduced risk of contaminating clean room environments.
It is important to achieve these objectives while retaining the flexible nature of the sheet material employed for the package or bag and while retaining the fully effective anti-static qualities of the bag both as to electrostatic buildup within the package and electrostatic field.